Spinal cord MRI at 7T

Abstract

Magnetic resonance imaging (MRI) of the human spinal cord at 7T has been demonstrated by a handful of research sites worldwide, and the spinal cord remains one of the areas in which higher fields and resolution could have high impact. The small diameter of the cord (∼1 cm) necessitates high spatial resolution to minimize partial volume effects between gray and white matter, and so MRI of the cord can greatly benefit from increased signal-to-noise ratio and contrasts at ultra-high field (UHF). Herein we review the current state of UHF spinal cord imaging. Technical challenges to successful UHF spinal cord MRI include radiofrequency (B₁) nonuniformities and a general lack of optimized radiofrequency coils, amplified physiological noise, and an absence of methods for robust B₀ shimming along the cord to mitigate image distortions and signal losses. Numerous solutions to address these challenges have been and are continuing to be explored, and include novel approaches for signal excitation and acquisition, dynamic shimming and specialized shim coils, and acquisitions with increased coverage or optimal slice angulations.

Figure 1

Comparison of 3T and 7T cervical spinal cord images at C5/C6. (A) 3T image acquired using the 19-channel commercial coil with 0.5 × 0.5 × 3 mm resolution (acquisition time = 1 min 44 sec). (B) 7T image acquired using the 19-channel receive array and four-channel transmit array (Zhao et al., 2014) with 0.5 × 0.5 × 3 mm resolution (acquisition time = 1 min 44 sec). (C) 7T image also acquired using the 19-channel receive array and four-channel transmit array with 0.3 × 0.3 × 3 mm resolution (no interpolation) (acquisition time = 3 min 43 sec). Reproduced from Zhao et al., 2014 with permission from John Wiley and Sons.

Figure 2

Montage of coils for human spinal cord MRI at 7T. (A) Volume transmit and 16-channel receive (manufactured by Quality Electrodynamics) for the cervical cord. Courtesy of Johanna Vannesjo; Oxford Centre for Functional MRI of the Brain, University of Oxford, Oxford, United Kingdom. (B) Four-channel transmit and 19-channel receive for the cervical cord. Courtesy of Lawrence Wald; Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA. (C) Eight-channel transceive (manufactured by Rapid Biomedical GmbH) for the cervical cord. Courtesy of Aurélien Massire and Virginie Callot; Center for Magnetic Resonance in Biology and Medicine, Aix-Marseille University, Marseille, France. (D) Quadrature transmit and 16-channel receive (manufactured by Nova Medical Inc.) for the cervical cord. Courtesy of Samantha By and Seth Smith; Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, USA. (E) Four-channel transmit and 22-channel receive (two panel wrap-around) for the cervical cord and brainstem. Courtesy of Alan Seifert, Bei Zhang, and Junqian Xu; Translational and Molecular Imaging Institute (TMII), Icahn School of Medicine at Mount Sinai, New York, NY, USA. (F) Flexible 8-channel transmit and 32-channel receive head and cervical spine coil, optimized for x–y and z B₁⁺ shimming. Note the full visual clearance and that the setup fits all head sizes. Courtesy of Dennis Klomp et al.; University Medical Center Utrecht and MR Coils, Utrecht, The Netherlands. (G) (i) Custom-built 32-channel transceive body coil, integrated between bore liner and gradient coil, for the thoracolumbar cord. (ii) Eight-channel flexible transceive body coil, based upon meander stripline elements, for the thoracolumbar cord. (iii) Eight-channel transceive coil, based upon rectangular loop elements, for the cervical cord. All images courtesy of Oliver Kraff and Stephan Orzada; Erwin L. Hahn Institute for Magnetic Resonance Imaging, Essen, Germany. (H) (i) Four-channel “loopole” transceive for the lumbar cord. (ii) Six-channel flexible transceive for the cervical cord. Both images courtesy of Karthik Lakshmanan; Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY, USA. (iii) Four-channel transceive (manufactured by Rapid Biomedical GmbH) for the cervical cord. Courtesy of Eric Sigmund; New York University Langone Medical Center, New York, NY, USA. (I) Graphical overlay of two electric dipole antennae (transmit) and eight loop coils (receive) for the thoracic cord. Courtesy of Qi Duan, Govind Nair, Natalia Gudino, Jacco A. de Zwart, Peter van Gelderen, Joe Murphy-Boesch, Daniel S. Reich, Jeff H. Duyn, and Hellmut Merkle; National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, USA. (J) Modular (top) 8-channel transceive array for the cervical cord and (bottom) 16-channel transceive array for the cervicothoracolumbar cord. Both arrays can be integrated in the patient table cushions and are duly approved by a notified body for implementation in clinical studies. Courtesy of Thoralf Niendorf, MRI.TOOLS GmbH, Berlin, Germany. (K) Butterfly-shaped coil for the lumbar cord. Two other coils with similar design have been constructed for the cervical and cervicothoracic cords, respectively. Courtesy of Junghwan Kim, Chan-Hong Moon, and Kyongtae T Bae; University of Pittsburgh, Pittsburgh, PA, USA. (L) Nonoverlapping microstrip transceive array, with induced current compensation or elimination (ICE) or magnetic wall decoupling (Li et al., 2011), for the thoracic cord. Courtesy of Xiaoliang Zhang; University of California, San Francisco; San Francisco, CA, USA. (M) Quadrature transmit and 8-channel receive for the cervicothoracolumbosacral cord. Courtesy of Andrew Webb; C.J. Gorter Center for High Field Magnetic Resonance Imaging, Leiden, The Netherlands.

Figure 3

B₀ shimming over the cervical spinal cord at 7T, using a shim volume covering the spinal cord from C1 to C7. (A) Anatomy of the spinal cord in a healthy volunteer. ΔB₀ field map (13 sagittal slices, in-plane resolution = 1×1 mm², slice thickness = 2 mm, TR = 620 ms, first echo time (TE) = 4.08 ms, ΔTE = 1.02 ms, flip angle = 53°) (B) before and (C) after 2^nd order B₀ shimming to compensate for magnetic field inhomogeneities within the spinal cord. The shimmed field map reveals residual quasi-periodic high-frequency field distortions along the cord near the intervertebral junctions.

Figure 4

Challenges of B₀ shimming at 7T. (A) Highly suboptimal automated shimming in the spinal cord at 7T results in unusable functional images. (B) The next run repeats the shimming procedure and achieves a good shim that produces excellent functional images. This extreme variability from one run to the next demonstrates a clear need for more robust spinal cord B₀ shimming at UHF.

Figure 5

Improved spinal cord acquisition strategies to be demonstrated at 7T. (A) Diffusion tensor imaging in the cervical spinal cord at 3T. (Left panel) Acquisition without SMS with coverage from C2 to C5. (Right panel) New DTI protocol with SMS illustrating the acquisition of twice as many slices in the same scan time. Note that in the SMS acquisition, the volume placement was raised by one vertebral level (centered on the C2/C3 junction) to cover the brainstem and entire cervical cord. (B) Optimal slice placements along the spinal cord. (Left panel) Placement of 12 5-mm functional slices centered on the C3/C4 junction in one healthy volunteer. This subject’s spinal cord is remarkably straight, which maintains perpendicularity between the slices and the cord and conveniently minimizes through-slice partial volume effects between white and gray matter. (Center panel) The identical placement in another volunteer illustrates curvature at C5 (which is common across subjects), and the challenge of acquiring slices that are consistently perpendicular to a structure that curves. (Right panel) A variable slice angulation scheme that maintains perpendicularity to the spinal cord shown in the center panel would have a variable and optimal angulation for each slice.

Figure 6

Visualization of MS lesions is enhanced in T₂*-weighted images at 7T compared to similar T₂*-weighted, or standard-of-care T₂-weighted, acquisitions at 3T (Dula et al., 2016a). Reproduced from Dula et al., 2016a under STM Permissions Guidelines with modifications approved by SAGE Publications Ltd.